The need for low emissions has created an increased demand for light olefins for use in alkylation, oligomerization, and MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand as feedstock for polypropylene production.
Light olefins, such as ethylene and propylene, can be produced by thermally cracking naphtha feedstocks containing paraffinic and isoparaffinic compounds, naphthenes and aromatics to produce light olefins. The thermal cracking of naphtha is carried out by exposing naphtha and steam to relatively high temperatures in the tubular coils of a fired heater. A problem associated with this technique is that the process is energy intensive, not very selective, produces coke, and releases significant amounts of carbon dioxide into the air.
Another technique for producing light olefins involves the catalytic cracking of hydrocarbons, such as naphtha or a butene-containing feed. In the catalytic cracking of naphtha or a butene-containing feed, the process is carried out by contacting the naphtha or butene-containing feed with a catalyst usually comprised of one or more crystalline microporous molecular sieves to selectively convert the feed into an olefin-containing mixture. Although various naphtha and butene catalytic cracking processes have been proposed in the past, many of the processes do not produce commercially important light olefins, e.g., propylene, with sufficient selectivity or yield. Also, the processes can produce undesirable amounts of methane and aromatics as unwanted byproducts. In contrast, a practical and economic butene or naphtha catalytic cracking process should selectively produce increased amounts of light olefins, e.g., propylene, while producing minimal amounts of methane and aromatics.
Fluid catalytic cracking (FCC) is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. Conventional processes for catalytic cracking of heavy hydrocarbon feedstocks to gasoline and distillate fractions typically use a catalyst containing a large pore molecular sieve, such as zeolite Y, as the primary cracking component and, optionally, an intermediate pore size molecular sieve, such as ZSM-5. While FCC is an efficient process for converting heavier feed to lighter products, many times the process makes less than desirable amounts of light olefins, e.g., propylene.
One problem which has persisted with molecular sieve, e.g., zeolite, catalysts is that of hydrothermal stability. In many of the processes in which they are used, the molecular sieves are exposed to water vapor at elevated temperatures and this tends to reduce the activity of the molecular sieve because of the loss of acidic sites through dehydroxylation and dealuminization, the loss being manifested by a decrease of the alpha value of the molecular sieve. The alpha value is a measure of the ability of the molecular sieve in cracking of paraffins. In some cases, particularly with zeolites having a low silica to alumina ratio, crystallinity may be adversely affected. Different molecular sieves exhibit different degrees of hydrothermal stability, but the problem is encountered to some extent with all of them. The exposure to the water vapor may occur during the catalytic process itself or in an ancillary treatment step. For example, in processes such as catalytic cracking, steam stripping, which is used to remove occluded hydrocarbons from the catalyst prior to regeneration, can cause a reduction of activity of the catalyst, as will any steam which is present in the regeneration and which has been produced either by combustion of any hydrocarbon material on the catalyst itself or by the combustion of hydrocarbon fuel used to heat the regenerator. The deleterious effect of the steam becomes more pronounced the longer and more frequent the exposure to it is; processes in which the catalyst is continuously or continually exposed to steam therefore present greater problems than those where the contact is occasional or at very long intervals. For instance, in FCC units the catalyst is continuously circulated through the reactor and the regenerator and comes into contact with steam during each complete cycle when the catalyst is subjected to the stripping and regeneration steps.
Attempts to improve the hydrothermal stability of molecular sieves have often been made and have met with varying success. Moreover, although it has often been found possible to improve the hydrothermal stability, other properties of the molecular sieve may be adversely affected. For example, the rare earth form of faujasite zeolite has improved hydrothermal stability, but the activity of the zeolite in the rare earth form may not be as great as it would be in other forms. Moreover, producing the rare earth form of a molecular sieve may not always be a practicable method to improve its hydrothermal stability. For example, rare earth cations do not readily enter the structure of some molecular sieves because of the low ion exchange selectivity of these cations.
The present invention provides catalysts having improved hydrothermal stability and processes for the catalytic cracking of hydrocarbon feedstocks, e.g. naphtha and heavier hydrocarbon feedstocks, which are effective in producing enhanced yields of propylene, as compared with known processes used to crack hydrocarbons.